With SPF 30 sunscreen, I am usually protected for an entire day and wont be getting more than a little darker. Now, I am getting low on SPF 30 cream but I have a large supply of SPF 15. Can I just use that one twice often to make up for the difference?
How does SPF 15 and SPF 30 sunscreen relate? Is it additive in strength or duration? Meaning can 15 be applied twice as often or twice as much? Or is the protection different?
Note that I am looking for how the various protection levels relate.